That simple but often overlooked fact prompted two-time entrepreneur and ad-tech veteran Josh Reznick to build Datalot, a "pay-per-call" platform. The company uses the data it gathers to identify the clicks most likely to turn into paying customers, figures out how much those clicks might be worth, and auctions off the leads in real time. "People shouldn't be buying clicks," said Mr. Reznick.

Publishers that host the ads like Datalot, too, because by using it, they can get three to five times the revenue the typical click generates. That price advantage, along with the explosion of mobile search, helped Datalot achieve a nearly 2,000% annual growth rate last year.

The company earned a slight profit in 2012, but for now, said Mr. Reznick, he's focused on growth, targeting new industries, such as home-security installers, and providing seamless Internet-to-phone service.